I have NetBeans 7.3.1 installed on mac (10.8.4) and it works flawlessly. However, I remember that I had a similar problem when trying to create a project in a directory that I thought was empty (it wasn’t ...). For me it's just a matter of deleting the project directory. I found the same answer for you while Googling as @striders. If you are using a mac, there will be no .netbeans folder containing "userdir"; rather, a directory ~/Library/Application\ Support/NetBeans/7.2. I understand that "7.2" is "userdir". Therefore, if the solution is for NetBeans to recreate this directory, simply delete it and restart. I tested the deletion of this folder, and NetBeans creates it at startup.
http://wiki.netbeans.org/FaqWhatIsUserdir (information in the catalog ...)
http://forums.netbeans.org/post-59080.html (- , ...)